The present invention relates to a transfer apparatus and a transfer method used in an image forming apparatus.
In an electrophotographic type copying apparatus or printer apparatus, a predetermined surface potential is imparted to a photosensitive body holding an electrostatic latent image so as to selectively change the surface potential of the photosensitive body corresponding to the background portion or the image portion, followed by supplying a toner (developing agent) to the surface of the photosensitive body so as to form a toner image (developing agent image) in the portion where the surface potential is selectively changed. The toner image thus formed is transferred onto a sheet-like material.
The toner image transferred onto the sheet-like material is melted and pressurized by a fixing apparatus so as to be fixed to the sheet-like material.
The method of transferring the toner image on the photosensitive body onto one surface of the sheet-like material can be roughly classified into a system of a non-contact type transfer bias voltage supply using, for example, a corona charger and another system of a contact transfer bias voltage supply using a roller, a brush, etc. In the case of using the contact type transfer bias voltage supply, it is possible to achieve a stable image transfer because the transfer bias voltage supply performs the function imparting a charge to the sheet-like material and the function of permitting the sheet-like material to be attached to the outer circumferential surface of the photosensitive body. Also, the system of using the contact type transfer bias voltage supply is advantageous over the system using the corona charger in that it is possible to suppress markedly the ozone generation.
Japanese Patent Publication (Kokoku) No. 62-24793 discloses a method of transferring a toner image from the photosensitive body onto a paper sheet while electrostatically holding the paper sheet on a transfer belt. Also, in the transfer system using a transfer belt, the paper sheet is held by the belt and, thus, the paper sheet is unlikely to be wound about the photosensitive body. Therefore the position of the paper sheet during transfer is unlikely to be changed. In other words, the pass of the paper sheet is stable. Under the circumstances, the transfer system using a transfer belt is absolutely required in the color image forming apparatus disclosed in, for example, Japanese Patent Publication No. 6-52446, in which four photosensitive bodies and four image forming sections for developing the latent image formed on the photosensitive bodies, which are for the four color components of yellow, magenta, cyan and black, respectively, are arranged in series.
The system using a transfer belt is also advantageous in a high speed printer or a high speed PPC having a high image forming capability for unit time in order to realize a stable transfer of a sheet-like material because the sheet-like material is unlikely to be wound about the photosensitive body. In this case, it is possible to ensure a stable transfer nip and to realize a good transfer by forming the transfer belt separately from the transfer bias voltage supply and by using a contact type transfer bias voltage supply as a device for imparting a transfer charge to the transfer belt.
On the other hand, in the case of using a contact type transfer bias voltage supply, it is desirable for the transfer bias voltage to be applied in only the case where the sheet-like material is present in the transfer nip. It should be noted in this connection, however, that it is unavoidable for the timing at which the sheet-like material arrives at the transfer nip to be changed by several milliseconds because of the nonuniformity of the transfer mechanism for transfer-ring the sheet-like material. Under the circumstances, if the sheet-like material arrives at the transfer nip with a delay from the theoretic timing, the transfer bias voltage is applied before the sheet-like material arrives at the transfer nip and, thus, the transfer bias is applied directly to the photosensitive body, though the transfer bias should desirably be applied to the photosensitive body through the sheet-like material. As a result, a strong charge of a polarity opposite to the charging polarity is radiated to the photosensitive body. Since the photosensitive body is not sensitive to the charge of the opposite polarity in almost all the cases, the charge is not erased in the charge eliminating process using an erasing lamp. It follows that the image forming process proceeds to the subsequent charging step and the light exposure step while retaining the charge of the opposite polarity.
Because of the presence of the charge of the opposite polarity, the photosensitive body is not charged sufficiently to a predetermined potential in the charging process. Alternatively, the photosensitive characteristics in the particular portion are changed. As a result, only that portion of the photosensitive body which has directly received the transfer electric field exhibits a concentration differing from that in the other portion in the intermediate concentration image represented by a half tone, giving rise to an undesired image called a memory image by a transfer bias voltage so as to impair the uniformity of the image.
The memory image by the transfer bias voltage appears prominently in the transfer system using the contact type transfer bias voltage supply.
However, where the transfer bias voltage is applied to the inside of the sheet-like material within the region of the sheet-like material, if the sheet-like material enters the transfer nip earlier only slightly than the design value, the image is not transferred in the reading edge portion, giving rise to the problem that the image is not formed in the reading edge portion.
As described above, it is very difficult to suppress the occurrence of the memory image by the transfer bias voltage while lowering the failure to form the image in the reading edge portion of the sheet-like material. Incidentally, a method of eliminating the memory image by the transfer bias voltage, in which the charge of the opposite polarity received by the photosensitive body while the sheet-like material passes through the transfer nip is eliminated by applying an AC voltage from an AC corona charger, has already been put to a practical use. However, in order to arrange the AC corona charger for the charge elimination purpose, it is necessary for the photosensitive body to have a large outer diameter. Where the photosensitive body has an outer diameter smaller than about 40 mm, it is physically impossible to arrange the particular AC corona charger.
As described above, in the image forming apparatus employing a photosensitive body of a small diameter by using a transfer mechanism using a contact type transfer bias voltage supply or a transfer mechanism using a transfer belt and a contact type transfer bias voltage supply (roller body) in combination, it is difficult to satisfy both the complete elimination of the memory image by the transfer bias voltage and the elimination of the failure to form an image in the reading edge portion. Incidentally, the similar phenomenon also takes place in the trailing edge portion of the sheet-like material. If the image in the trailing edge portion of the sheet-like material is to be transferred, it is unavoidable for the timing of stopping the supply of the transfer bias voltage by the transfer bias supply device to be positioned outside the sheet-like material, giving rise to the problem that the surface potential of the photosensitive body is adversely affected (the surface potential of the photosensitive body is partially changed). In this case, if the distance between the adjacent sheet-like materials is larger than the length of one complete rotation of the photosensitive body (outer circumferential length of the photosensitive body), the surface potential of the photosensitive body is made substantially uniform by the steps of the charging and the charge elimination, with the result that the surface potential is brought back to the predetermined charging potential in the subsequent charging step. However, where the distance between the adjacent sheet-like materials is smaller than the outer circumferential length of the photosensitive body, the memory image by the transfer bias voltage is generated in the reading edge portion of the next image. It follows that, in the image forming apparatus in which the distance between the adjacent sheet-like materials is smaller than the outer circumferential length of the photosensitive body, it is impossible to prevent the failure of transfer in the trailing edge portion of the image while suppressing the generation of the memory image by the transfer bias voltage.
An object of the present invention is to provide an image forming apparatus that permits suppressing the occurrence of a memory image by the transfer bias voltage while suppressing the failure to form an image in the tip end or both the tip end and the trailing edge portion when a continuous print out is obtained in an image forming apparatus in which the distance between the adjacent sheet-like materials is smaller than the outer circumferential length of the photosensitive body.
Another object of the present invention is to provide an image forming apparatus that permits suppressing the occurrence of a memory image by the transfer bias voltage while suppressing the failure to form an image in the tip end or both the tip end and the trailing edge portion in an image forming step applied to a sheet-like material having a narrow range of an appropriate value of the transfer bias voltage.
According to a first aspect of the present invention, there is provided an image forming apparatus, comprising an image forming section for forming a toner image on an image carrier; a transfer device that is brought into contact with the image carrier with a transfer medium interposed therebetween, the transfer medium including at least one of paper sheets and sheet-like resins, for transferring the toner image formed by the image forming section onto the transfer medium; a bias voltage supply for supplying a transfer bias voltage to the transfer device; and a bias voltage controller for switching the on-off state of the bias voltage output from the bias voltage supply and the magnitude and polarity of the bias voltage into a predetermined magnitude and polarity at a predetermined timing in accordance with the presence and absence of the transfer medium.
According to a second aspect of the present invention, there is provided an image forming apparatus, comprising an image forming section for forming a toner image on an image carrier; a transfer device that is brought into contact with the image carrier with a transfer medium interposed therebetween, the transfer medium including at least one of paper sheets and sheet-like resins, for transferring the toner image formed by the image forming section onto the transfer medium; a bias voltage supply for supplying a transfer bias voltage to the transfer device; a device for changing the transfer interval of the transfer media for changing the timing of guiding the transfer media toward the image forming section; and a bias voltage controller for switching the on-off state of the bias voltage output from the bias voltage supply and the magnitude and polarity of the bias voltage into a predetermined magnitude and polarity at a predetermined timing in accordance with the presence and absence of the transfer medium.
According to a third aspect of the present invention, there is provided an image forming apparatus, comprising an image forming section, including an image carrier, for forming a toner image on the image carrier; an image carrier rotating device for rotating the image carrier in the image forming section so as to move the outer circumferential surface of the image carrier at any of a first speed and a second speed lower than the first speed; a transfer device that is brought into contact with the image carrier with a transfer medium interposed therebetween, the transfer medium including at least one of paper sheets and sheet-like resins, for transferring the toner image formed on the image carrier in the image forming section onto the transfer medium; a transfer medium transfer device for transferring the transfer medium at any of a first speed equal to the speed at the outer circumferential speed of the image carrier and a second speed lower than the first speed; a speed changing device for changing the speed of each of the image carrier rotating device and the transfer medium transfer device to the second speed in forming a toner image on the sheet-like resin; a bias voltage supply for supplying a transfer bias voltage to the transfer device; and a bias voltage controller for switching the on-off state of the bias voltage output from the bias voltage supply and the magnitude and polarity of the bias voltage into a predetermined magnitude and polarity at a predetermined timing in accordance with the presence and absence of the transfer medium.
Further, according to a fourth aspect of the present invention, there is provided an image forming apparatus, comprising an image forming section for forming a toner image on an image carrier; a transfer device that is brought into contact with the image carrier with a transfer medium interposed therebetween, the transfer medium including at least one of paper sheets and sheet-like resins, for transferring the toner image formed by the image forming section onto the transfer medium; a bias voltage supply for supplying a transfer bias voltage to the transfer device; an environmental state detecting device for detecting at least one of temperature and humidity in the vicinity of the image forming section; and a bias voltage controller for switching the on-off state of the bias voltage output from the bias voltage supply and the magnitude and polarity of the bias voltage into a predetermined magnitude and polarity at a predetermined timing in accordance with the presence and absence of the transfer medium, the kind of the transfer medium and the environmental condition detected by the environmental state detecting device.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.